A known two-stage submerged vinegar fermentation process and the associated equipment for producing vinegar with an acetic acid concentration of more than 15 g/100 ml are described in Austrian Pat. No. 363,053. A description of that same process, but without a description of the equipment details, is also found in U.S. Pat. No. 4,076,844. In that process, during a first fermentation stage the total concentration (the sum of the alcohol and acetic acid concentrations, with alcohol concentration expressed in volume by volume and acetic acid concentration expressed in weight by volume) of the liquid in a main fermentation tank is increased to more than 15% from a starting value in the range of 12-15% at an alcohol content of from 1-5 vol. % by means of a slow addition of alcohol and in particular while the acetic acid concentration is limited to at most 15 g/100 ml. Thereafter, a part of the substrate is discharged from the main fermentation tank into a secondary fermentation tank in order to continue the fermentation, while the total concentration is kept constant, until the alcohol concentration has decreased to less than 0.5 vol. %.
This process is based on the recognition that the vinegar bacteria (Acetobacter), even after having lost the capacity for multiplication, are still capable, for a limited period of time, to support acidification, i.e., to oxidize alcohol into vinegar. In the known process, this capability, which depends on the existence of certain conditions, is put to use in the secondary fermentation tank, while in the main fermentation tank the multiplication capacity of the vinegar bacteria, which is required for the continuation of the fermentation, is maintained through a limiting of the acetic acid concentration. The quantity of substrate discharged from the main fermentation tank into the secondary fermentation tank is replaced through an injection of fresh mash into the main tank, by means of which the original starting concentrations of acetic acid and alcohol are again attained. As a consequence, in the main fermentation tank the fermentation can be repeated under conditions which are the same as before and until, through the gradual influx of alcohol to the extent of the alcohol oxidation, the total concentration of the substrate is again increased to above 15 % while the acetic acid concentration is concurrently limited to remain below 15 g/100 ml. At that point in time, a portion of the substrate in the main fermentation tank is again discharged therefrom and transferred into the secondary fermentation tank, after which the above-described fermentation proceeds anew.
As will be apparent from the description of the known process in the aforesaid Austrian patent, in that process the feed of alcohol into the main fermentation tank is controlled by a device which analyzes the alcohol concentration in the main tank, during which analysis the alcohol concentration as measured is compared with a predetermined desired value thereof. If the alcohol concentration sinks below the predetermined desired value, then the feed pump for the alcohol is activated until the desired value is again reached. Despite this control of the alcohol feed pump in dependence on the alcohol concentration in the main fermentation tank, the effort expended to assure an undisturbed running of the process is relatively great, because the acetic acid concentration in the main fermentation tank must be limited and care must be taken to achieve a properly timed transfer of a portion of the substrate from the main fermentation tank into the secondary fermentation tank. Thus, automatic monitoring and control is needed for the entire process.
A known single-stage fermentation process for producing vinegar with an acetic acid concentration of above 15 g/100 ml in a fermentation tank is described in German published patent application No. 3,514,634. In that process, the addition of alcohol to a mash having predetermined starting concentrations of acetic acid and alcohol is controlled in dependence on the instantaneous extent of the oxidation of the alcohol to vinegar. To this end, the acetic acid concentration on the one hand and the alcohol concentration on the other hand are periodically measured, and from the change of the acetic acid concentration per unit of time the amount of alcohol to be added over each unit of time, under the assumption of a constant increase in the acetic acid concentration up to a predetermined end concentration, is determined in a computer. With the help of the computer, the calculated desired value of the alcohol feed rate is compared with the measured actual value of the added quantity of alcohol in order to match the added quantity of alcohol to the desired value of the alcohol concentration through a balancing of the desired and actual values thereof. Apart from the fact that such a control of a single-stage fermentation process is not suited for use in a two-stage fermentation process, it suffers from the disadvantage that by virtue of the continuous matching of the added quantity of alcohol to the measurement of the instantaneous increase of the acetic acid concentration a continuous variation in the alcohol concentration is obtained, which adversely affects the vinegar fermentation.